1. Technical Field of Invention
The present invention relates to an inexpensive, disposable, irreversible indicator suitable for providing visual indication of exposure to a selected temperature of the application of a predetermined force or pressure to a package or product.
2. Description of the Prior Art
Various devices used to indicate visually the passage of a predetermined interval of time or to signal if frozen food packages have been subjected to defrosting conditions prior to reaching the consumer are known in the prior art. Some of the chemical timing devices which are known have involved the exposure of a wicking material to a free liquid source such as a pool of liquid, and have provided a chemical agent in the wick at some predetermined point which reacted with the liquid migrating along the wick to create a color change. Some devices have incorporated a porous, fibrous material saturated with a liquid as a substitute for the free pool to provide improved liquid containment and facilitate storage of the device. Other devices have incorporated an impervious barrier strip between the porous liquid containment pad and the wick to facilitate storage by preventing contact between the pad and the wick prior to intentional activation by removal of the impervious strip.
One such device is shown in U.S. Pat. No. 3,962,920, issued June 15, 1976 to Manske, and assigned to the assignee of the present invention, which teaches a wick superimposed over a porous pad containing a dyed chemical and having an impervious barrier imposed therebetween. Removal of the barrier permits migration of the dyed chemical from the pad to the wick and movement along the wick indicates a time interval. Further, a critical temperature indication is provided by impregnating the pad with a dye in solution with a chemical which remains solid until a particular temperature is reached. Although the device is activated by removal of the impervious barrier strip, a visual indication is not provided until the critical temperature is reached and the chemical becomes liquid, allowing the dyed chemical to migrate into the wick.
Another such device is shown in U.S. Pat. No. 3,954,011 issued May 4, 1976 to Manske, also assigned to the assignee of the present invention, which shows a resilient carrier which maintains the chemically impregnated pad separate from the wick, thus eliminating the need for an impervious barrier. The device will remain unactivated and liquid migration prevented until the wick is physically forced into contact with the pad and retained in such a relationship by the carrier.
Indicating devices which use free pools of liquid as a fluid source have many disadvantages. The use of a free pool of liquid inherently creates problems related to containing the liquid pool and maintaining separation of the liquid from the wick prior to activating the device, for example when shipping a supply of indicators. Provision of a liquid containing pad and imposition of a barrier strip between the liquid pad and the wick have proven expedient, however, these devices increase manufacturing and assembly costs Devices which maintain the liquid pad and the wick in a spaced relationship, such as the aforementioned U.S. Pat. No. 3,954,011, have succeeded in eliminating the impervious barrier strip, but have required relatively expensive means to do so.